SEVERAL cases and small series of cases of cyst of the semilunar cartilage of the knee have been reported, but no large series has been presented at any time. This benign tumor has been seen rather infrequently. Blanco and Postoloff1 reviewed the literature extensively and concluded that it was first described by Nicasse in 1883. Ghormley and Dockerty2 reported four cases from this institution in 1943. The cause and origin of the tumor still are poorly understood, and much is left to speculation. Our purpose is to report our findings in 50 patients seen in 28 years at one institution, and to stress that adequate treatment includes removal of the semilunar cartilage as well as the cyst. Material and Method We reviewed the records of all cases from 1935 through 1962 in which treatment was given at the Mayo Clinic for cyst of the semilunar cartilage of the